Glass composition, glass composition for chemical strengthening, strengthened glass article, and cover glass for display

ABSTRACT

Provided is a glass composition containing, in mol %: 56 to 66% SiO 2 ; 6 to 12% Al 2 O 3 ; 5 to 14% MgO; 0 to 1% CaO; 17 to 24% Na 2 O; and 0 to 3% K 2 O. The total content of Li 2 O, Na 2 O, and K 2 O is in a range of 18.5 to 24%. This glass composition is suitable for production by a float process and for introduction of a compressive stress layer with a high crack initiation load, a high surface compressive stress, and a large thickness by chemical strengthening.

TECHNICAL FIELD

The present invention relates to a glass composition suitable for chemical strengthening, more specifically to a glass composition having properties suitable for use as a cover glass of a display. The present invention also relates to a glass composition for chemical strengthening, a chemically-strengthened strengthened glass article, and a cover glass for a display.

BACKGROUND ART

In recent years, electronic devices with liquid crystal displays, organic EL displays, etc. and electronic devices with touch panel displays have been widespread. Since glass materials have high surface hardness, they are widely used as materials of cover glasses of displays of these electronic devices. Cover glasses of displays are sometimes chemically strengthened to improve their mechanical strength.

Chemical strengthening is a technique of replacing alkali metal ions contained in the glass surface by monovalent cations having a larger ionic radius so as to form a compressive stress layer on the glass surface. Chemical strengthening is often performed by replacing lithium ions (Li⁺) by sodium ions (Na⁺) or by replacing sodium ions by potassium ions (K⁺).

A glass composition suitable for chemical strengthening disclosed in Patent Literature 1 contains 64 to 68 mol % SiO₂, 12 to 16 mol % Na₂O, and 8 to 12 mol % Al₂O₃. In this glass composition, the content of Na₂O is higher than that of Al₂O₃ by 2 to 6 mol %, and the total content of alkaline earth metal oxides (MgO+CaO+SrO) is adjusted to 5 to 8 mol % (claim 1). In addition, the glass composition described in Patent Literature 1 has a melting temperature of less than 1650° C. and a liquidus viscosity of at least 13 kPa·s to be adapted to a down-draw process. In the glass compositions described as examples in Patent Literature 1, the contents of Al₂O₃ and Na₂O are 8.9 mol % or more and 14.38 mol % or less, respectively.

A strengthened glass substrate suitable for use in a touch panel display disclosed in Patent Literature 2 contains, in mass %, 45 to 75% SiO₂, 1 to 30% Al₂O₃, 0 to 20% Na₂O, and 0 to 20% K₂O (claim 3). Furthermore, in the examples, the temperatures at which the glass substrates have a viscosity of 10⁴ dPa·s are 1122° C. to 1414° C. These glass substrates are suitable for production by a down-draw process.

A working temperature and a melting temperature are known measures of the high-temperature viscosity of glass. In a float process, the working temperature is a temperature at which glass has a viscosity of 10⁴ dPa·s, and hereinafter may be referred to as T₄. The melting temperature is a temperature at which glass has a viscosity of 10² dPa·s, and hereinafter may be referred to as T₂.

CITATION LIST Patent Literature

Patent Literature 1: JP 2010-527892 T

Patent Literature 2: JP 2010-116276 A

SUMMARY OF INVENTION Technical Problem

Glass compositions having a low T₂ and a low T₄, in particular, glass compositions having a low T₄ are suitable for production by the float process. On the other hand, glass compositions for chemical strengthening are expected to be susceptible to ion exchange by replacement of cations so as to introduce a compressive stress layer with a high crack initiation load, a high surface compressive stress, and a large thickness.

In view of the above circumstances, it is an object of the present invention to provide a glass composition suitable for production by a float process and for introduction of a compressive stress layer with a high crack initiation load, a high surface compressive stress, and a large thickness by chemical strengthening.

Solution to Problem

In order to achieve the above object, the present invention provides a glass composition containing, in mol %: 56 to 66% SiO₂; 6 to 12% Al₂O₃; 5 to 14% MgO; 0 to 1% CaO; 17 to 24% Na₂O; and 0 to 3% K₂O. The total content of Li₂O, Na₂O, and K₂O is in a range of 18.5 to 24%.

In another aspect, the present invention provides a strengthened glass article including a compressive stress layer formed as a surface of the strengthened glass article by bringing a glass article containing the glass composition of the present invention into contact with a molten salt containing monovalent cations having an ionic radius larger than that of sodium ions so as to allow ion exchange to take place between sodium ions contained in the glass composition and the monovalent cations.

The present invention further provides a cover glass for a display, the cover glass including the strengthened glass article of the present invention.

Advantageous Effects of Invention

The glass composition according to the present invention has a relatively low T₄ and thus is suitable for production by the float process. Furthermore, the glass composition of the present invention is suitable for obtaining a strengthened glass article having a compressive stress layer with a high crack initiation load, a large thickness, and a high surface compressive stress.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the percentages of the components of glass compositions are all expressed in mol %, unless otherwise specified. In this description, the phrase “consisting essentially of components” means that the total content of the components referred to is 99.5 mol % or more, preferably 99.9 mol % or more, and more preferably 99.95 mol % or more. The phrase “being substantially free of a component” means that the content of the component is 0.1 mol % or less, and preferably 0.05 mol % or less.

The glasses disclosed in Patent Literature 1 and Patent Literature 2 have high high-temperature viscosities and high T₄ values. A high T₄ value is disadvantageous in producing a cover glass of a display by the float process, and is also disadvantageous in forming glass into a thin sheet as a cover glass of a display.

The present invention is intended to reduce the T₄ by a thorough study of, in particular, the contents of Al₂O₃, Na₂O, alkaline earth oxides, and alkali metal oxides in view of the effects of each of these oxides on the properties and by an overall adjustment of the contents of the other components, and thereby to provide a glass composition suitable for production by the float process, in particular, a glass composition advantageous in forming glass into a thinner sheet (for example, with a thickness of 1 mm or less) as a cover glass for a display and resistant to scratching and cracking.

The following points are not essential in the present invention. However, in the present invention, attention can be given to the following points in some cases.

The present invention is intended to provide a glass composition having a relatively low T₂ so as to be adapted to a glass melting furnace used in conventional production facilities for the float process. The present invention is also intended to provide a glass composition in which a value obtained by subtracting the liquidus temperature TL from the T₄ is a not too large negative value or a positive value (for example, −10° C. or more, preferably 0° C. or more, and more preferably 10° C. or more) so as to be adapted to glass formation by the float process.

Hereinafter, the components of the glass composition of the present invention are described respectively.

(SiO₂)

SiO₂ is the main component of a glass composition. An excessively low content of SiO₂ reduces the chemical durability such as water resistance and heat resistance of the glass. On the other hand, an excessively high content of SiO₂ increases the viscosity of the glass composition at high temperatures and thus makes it difficult to melt and form the glass composition. Therefore, the appropriate content of SiO₂ is in a range of 56 to 66%. The content of SiO₂ is preferably 57 to 64%, and more preferably 57 to 62%.

(Al₂O₃)

Al₂O₃ improves the chemical durability such as water resistance of a glass composition and further facilitates migration of alkali metal ions in the glass. Al₂O₃ is also a component that contributes to maintaining the strength obtained by chemical strengthening. On the other hand, an excessively high content of Al₂O₃ increases the viscosity of the glass melt, and thus increases the T₂ and T₄ and reduces the clarity of the glass melt, which makes it difficult to produce a high quality glass sheet.

Therefore, the appropriate content of Al₂O₃ is in a range of 6 to 12%. The content of Al₂O₃ is preferably 11% or less, and more preferably 10% or less. The content of Al₂O₃ is preferably 7% or more, and more preferably 8% or more.

As for the contents of SiO₂ and Al₂O₃, the glass compositions close to and including the glass composition of the present invention have the following features.

In order to produce a glass sheet by the float process and to perform chemical strengthening treatment at a relatively low temperature and in a short time, a glass composition needs to contain a certain amount of Na₂O. Therefore, the glass composition of the present invention contains 17% or more Na₂O.

On the other hand, it was found that the glass compositions can be classified into two groups according to the following parameters of glass articles obtained by chemical strengthening: the surface compressive stress and the depth of the compressive stress layer; and the crack initiation load defined as an indentation load at which cracks emanating from an indentation formed by a Vickers indenter occur with a probability of 50%.

The first group of glass compositions include those that can provide a surface compressive stress of 750 MPa or more after any of chemical strengthening treatments that were performed thereon. In the first group of glass compositions, there is no correlation between the surface compressive stress and the depth of the compressive stress layer and the crack initiation load of the chemically-strengthened glass article, which reveals that the crack initiation load of the glass article is determined solely by the glass composition thereof.

The second group of glass compositions include those that can provide a surface compressive stress of less than 750 MPa after any of chemical strengthening treatments that were performed thereon. In the second group of glass compositions, there is a strong positive correlation between the crack initiation load of the chemically-strengthened glass article and the surface compressive stress, and thus the crack initiation load decreases rapidly as the surface compressive stress decreases. In addition, in all the glass compositions belonging to the second group, the crack initiation load had very low values.

In the present invention, based on the above-described findings, a glass composition belonging to the above-mentioned first group and having a SiO₂ content of 66% or less and an Al₂O₃ content of 6% or more was selected. With the use of the glass composition of the present invention in which the contents of SiO₂ and Al₂O₃ satisfy the above conditions and the contents of the other components are appropriately adjusted, it is possible to obtain a strengthened glass article having a compressive stress layer with a surface compressive stress of 900 MPa or more, a crack initiation load of 3.9 kgf or more, and a thickness of 25 μm or more, and even a strengthened glass article having a compressive stress layer with a surface compressive stress of 1000 MPa or more and a depth of 30 μm or more.

However, there is a positive correlation between the difference obtained by subtracting the content of Al₂O₃ from the content of SiO₂ (Si₂—Al₂O₃) and acid resistance of the glass composition. When a glass article having a low acid resistant glass composition is immersed in an acid solution such as an aqueous hydrofluoric acid solution, the surface of the glass article is damaged, regardless of whether it is subjected to chemical strengthening treatment or not. In view of this, in the glass composition of the present invention, the content of SiO₂ is 56% or more and the content of Al₂O₃ is 12% or less.

(Na₂O)

Na₂O is a component that increases the surface compressive stress and thus increases the depth of the surface compressive stress layer when sodium ions are replaced by potassium ions. However, if the content of Na₂O is higher than the appropriate content thereof, the surface compressive stress relaxed in the chemical strengthening treatment is greater than the stress produced by ion exchange in the chemical strengthening treatment, and as a result, the surface compressive stress is likely to decrease.

Na₂O is a component that increases the meltability and reduces the T₄ and T₂. On the other hand, an excessively high content of Na₂O significantly reduces the water resistance of glass.

Therefore, in the glass composition of the present invention, the appropriate content of Na₂O is in a range of 17 to 24%. The content of Na₂O is preferably 18.5% or more, and more preferably 19% or more. The content of Na₂O is preferably 22% or less, and more preferably 21% or less. However, in order to ensure reduction of the T₄, etc., the content of Na₂O may be 22% or more depending on the contents of the other components.

(MgO)

MgO is most effective in increasing the meltability of glass in alkaline earth oxides (RO components). In order to obtain this effect sufficiently, in the glass composition of the present invention, the content of MgO is 5% or more. On the other hand, an excessively high content of MgO beyond its appropriate content rapidly reduces the strengthening effects obtained by chemical strengthening, in particular, the depth of the surface compressive stress layer, and also reduces the crack initiation load. Among the RO components, MgO is least likely to have these negative effects, but in the glass composition of the present invention, the content of MgO is 14% or less. In addition, a high content of MgO increases the liquidus temperature TL of the glass composition.

Therefore, in the glass composition of the present invention, the content of MgO is in a range of 5 to 14%. The content of MgO is preferably 7% or more, and more preferably 8% or more. The content of MgO is preferably 12% or less, and more preferably 11% or less.

(CaO)

CaO has the effect of reducing the viscosity at high temperatures. However, an excessively high content of CaO inhibits migration of sodium ions in a glass composition and makes the glass composition more susceptible to devitrification. However, it is preferable to add CaO because a small amount of CaO is effective in lowering the liquidus temperature TL.

Therefore, the appropriate content of CaO is in a range of 0 to 1%. The content of CaO is preferably 0.7% or less, and more preferably 0.5% or less. The content of CaO may be 0.3% or more.

(SrO and BaO)

SrO and BaO are more effective than CaO in significantly reducing the viscosity of a glass composition and reducing the liquidus temperature TL of the glass composition, if their contents are low. Even if the contents of SrO and BaO are very low, they significantly inhibit migration of sodium ions in the glass composition and has a significant negative effect on both the surface compressive stress and the depth of the compressive stress layer.

Therefore, it is preferable that the glass composition of the present invention be substantially free of SrO and BaO.

(K₂O)

Like Na₂O, K₂O is a component that increases the meltability of glass. A low content of K₂O increases the ion exchange rate in chemical strengthening, increases the depth of the compressive stress layer, and at the same time lowers the devitrification temperature TL of a glass composition. Therefore, it is preferable that the glass composition have a low content of K₂O.

On the other hand, K₂O impairs the clarity of glass melt compared to Na₂O. An excessively high content of K₂O is more likely to reduce the crack initiation load after chemical strengthening. In addition, as the content of K₂O increases, a molten salt used in chemical strengthening decomposes and the chemical strengthening effect is more likely to decrease accordingly.

Therefore, the appropriate content of K₂O is in a range of 0 to 3%. The content of K₂O is preferably 1.5% or less, and more preferably 1% or less. The content of K₂O may be 0.2% or more, and even 0.5% or more.

(Li₂O)

Li₂O significantly reduces the depth of a compressive stress layer even if the content of Li₂O is very low. When a glass article containing Li₂O is subjected to chemical strengthening treatment in a molten salt of potassium nitrate alone, the molten salt decomposes at a much higher rate than in the case of a glass article free of Li₂O. Specifically, in the case where the chemical strengthening treatment is performed repeatedly using the same molten salt, the quality of the properties obtained by chemical strengthening degrade more rapidly, that is, the quality of the properties obtained degrade with fewer repetitions of the treatment. Therefore, it is preferable that the glass composition of the present invention be substantially free of Li₂O.

(R₂O)

R₂O refers to Li₂O, Na₂O, and K₂O. If the content of R₂O is too low, the amount of the components that reduce the viscosity of a glass composition is too small, which makes it difficult to melt the glass composition. On the other hand, when the glass composition of the present invention is subjected to chemical strengthening, ions derived from the molten salt diffuse into the glass to produce a compressive stress, but the compressive stress is likely to relax and decrease due to the relaxation of the glass structure, depending on the balance between the content of R₂O and the contents of Al₂O₃ and MgO. In order to minimize this negative effect, the upper limit is put on the content of R₂O in the glass composition of the present invention.

Therefore, the appropriate content of R₂O (appropriate total content of Li₂O, Na₂O, and K₂O) is in a range of 18.5 to 24%. The content of R₂O is preferably 19% or more, and preferably 22% or less. However, in order to ensure reduction of the T₄, etc., the content of R₂O may be 22% or more depending on the contents of the other components.

(B₂O₃)

B₂O₃ is a component that reduces the viscosity of a glass composition and improves the meltability thereof. However, an excessively high content of B₂O₃ makes the glass composition more susceptible to phase separation and reduces the water resistance of the glass composition. In addition, compounds formed from B₂O₃ and alkali metal oxides may vaporize and damage the refractory material of a glass melting chamber. Furthermore, the addition of B₂O₃ causes a decrease in the depth of the compressive stress layer formed by chemical strengthening. Therefore, the appropriate content of B₂O₃ is 3% or less. In the present invention, it is more preferable that the glass composition be substantially free of B₂O₃.

(Fe₂O₃)

Fe is normally present in the form of Fe²⁺ or Fe³⁺ in glass. Fe³⁺ is a component that improves the ultraviolet ray absorbing properties of glass, and Fe²⁺ is a component that improves the heat ray absorbing properties of glass. However, when the glass composition is used for a cover glass of a display, it is preferable to minimize the content of Fe to prevent the glass composition from being conspicuously colored. Fe may be inevitably mixed in the glass composition due to an industrial raw material, but it is recommended that the content of total iron oxide be 0.1% or less, and preferably 0.02% or less, as calculated in terms of Fe₂O₃ content. In the present invention, the glass composition may be substantially free of iron oxide.

(TiO₂)

TiO₂ is a component that reduces the viscosity of a glass composition and increases the surface compressive stress produced by chemically strengthening. However, a high content of TiO₂ colors the glass composition yellow, which is not desired. Therefore, the appropriate content of TiO₂ is 0 to 1%. There may be a case where TiO₂ is inevitably mixed in the glass composition due to an industrial raw material and the glass composition contains 0.05% TiO₂, but this low content of TiO₂ does not cause undesirable coloring.

(ZrO₂)

ZrO₂ is a component that increases the water resistance of glass and further increases the surface compressive stress produced by chemically strengthening. However, a high content of ZrO₂ may cause a rapid increase in the liquidus temperature TL. Therefore, the appropriate content of ZrO₂ is 0 to 1%. In the present invention, the glass composition may be substantially free of ZrO₂.

(SO₃)

In the float process, a sulfate such as sodium sulfate (Na₂SO₄) is widely used as a refining agent. A sulfate is decomposed in molten glass to produce a gas component, which promotes degassing of the glass melt, but a portion of the gas component is dissolved in the form of SO₃ and remains in the glass composition. In the glass composition of the present invention, the content of SO₃ is preferably 0.1 to 0.3%.

(SnO₂)

It is known that, in formation of a glass sheet by the float process, molten tin in a tin bath diffuses into the glass in contact with the tin bath so as to be present in the form of SnO₂. SnO₂ also contributes to degassing when it is mixed as one of the glass raw materials. In the glass composition of the present invention, the content of SnO₂ is preferably 0 to 0.4%.

(Other Components)

Preferably, the glass composition of the present invention consists essentially of the components (from Al₂O₃ to SnO₂) mentioned above. The glass composition of the present invention may contain components other than the above-mentioned components. In this case, the content of each of the other components is preferably less than 0.1%.

Examples of the other components that the glass composition may contain include As₂O₅, Sb₂O₅, CeO₂, Cl, and F in addition to the above-mentioned SO₃ and SnO₂. These components are added to degas the molten glass. However, it is preferable not to add As₂O₅, Sb₂O₅, Cl, and F because they have serious adverse effects on the environment. Other examples of the components that the glass composition may contain include ZnO, P₂O₅, GeO₂, Ga₂O₃, Y₂O₃, and La₂O₃. The glass composition may contain components other than the above-mentioned components derived from industrially available raw materials, unless the content of each of these components exceeds 0.1%. Since these components are optionally added if necessary or are inevitably mixed, the glass composition of the present invention may be substantially free of these components.

Hereinafter, the properties of the glass composition of the present invention are described.

(Glass Transition Temperature: Tg)

According to the present invention, it is possible to provide a glass composition having a glass transition temperature (Tg) of 610° C. or less, further 590° C. or less, or even 570° C. or less in some cases, and thus it is easier to slowly cool molten glass to produce the glass composition. The lower limit of the glass transition temperature is not particularly limited, and it may be 530° C. or more, preferably 550° C. or more to prevent relaxation of the compressive stress produced by ion exchange.

(Working Temperature: T₄)

In the float process, the viscosity of molten glass is adjusted to about 10⁴ dPa·s (10⁴ P) when the molten glass in a melting furnace is poured into a float bath. In the production by the float process, it is preferable that the temperature (working temperature: T₄) at which the molten glass has a viscosity of 10⁴ dPa·s be lower. For example, in order to form the glass into a thin sheet for use as a cover glass of a display, the working temperature T₄ of the molten glass is preferably 1100° C. or less. According to the present invention, it is possible to provide a glass composition having a T₄ of 1090° C. or less, further 1075° C. or less, or even 1060° C. or less in some cases and thus suitable for production by the float process. The lower limit of the T₄ is not particularly limited, and it is 1000° C., for example.

(Melting Temperature: T₂)

When the temperature (melting temperature: T₂) at which the molten glass has a viscosity of 10² dPa·s is low, the amount of energy required to melt the glass raw materials can be reduced, and the glass raw materials can be more easily dissolved to promote degassing and refining of the glass melt. According to the present invention, it is possible to reduce the T₂ to 1550° C. or less, and even 1530° C. or less.

(Difference Between Working Temperature and Liquidus Temperature: T₄-TL)

In the float process, it is preferable that molten glass does not devitrify when the temperature of the molten glass is T₄. In other words, it is preferable that the difference between the working temperature (T₄) and the liquidus temperature (TL) be large. According to the present invention, it is possible to provide a glass composition in which a difference obtained by subtracting the liquidus temperature from the working temperature is as large as −10° C. or more, and even 0° C. or more. In addition, according to the present invention, it is possible to reduce the TL to 1050° C. or less, and even 1000° C. or less so as to contribute to increasing the difference T₄−TL.

(Density (Specific Gravity): d)

It is desirable that a cover glass of a display for an electronic device have a low density to reduce the weight of the electronic device. According to the present invention, it is possible to reduce the density of the glass composition to 2.53 g·cm⁻³ or less, further 2.51 g·cm⁻³ or less, and even 2.50 g·cm⁻³ or less in some cases.

In the float process or the like, when production of glass is changed from one type of glass to another type of glass, if there is a large difference in the density between these two types of glass, a portion of one type of glass having a higher density melts and remains at the bottom of a melting furnace, which may affect the changeover to production of another type of glass. The density of soda lime glass, which is currently mass-produced by the float process, is about 2.50 g·cm⁻³. Therefore, for the mass production by the float process, it is preferable that the glass composition has a density close to the value mentioned above. Specifically, the density of the glass composition is preferably 2.45 to 2.55 g·cm⁻³, and particularly preferably 2.47 to 2.53 g·cm⁻³.

(Elastic Modulus: E)

When a glass substrate is subjected to chemical strengthening by ion exchange, it may be bent. It is preferable that the glass composition have a high elastic modulus to reduce this bending. According to the present invention, it is possible to increase the elastic modules (Young's modulus: E) of the glass composition to 70 GPa or more, and even to 72 GPa or more.

(Thermal Expansion Coefficient: α)

According to the present invention, it is possible to provide a glass composition having a linear thermal expansion coefficient in a range of 95×10⁻⁷/° C. to 112×10⁻⁷/° C. in a temperature range of 50 to 350° C. The glass composition having a linear thermal expansion coefficient in this range has the advantage of being less susceptible to bending or distortion when it is attached to a material having a higher linear thermal expansion coefficient than the linear thermal expansion coefficients (70×10⁻⁷/° C. to 100×10⁻⁷/° C.) of common glass members. According to a preferred embodiment of the present invention, it is possible to provide a glass composition having a linear thermal expansion coefficient in a range of 100×10⁻⁷/° C. or more in a temperature range of 50 to 350° C.

(Crack Initiation Load: Rc)

Cover glasses of displays are expected to be resistant to scratching and cracking. For the glass composition of the present invention, the crack initiation load determined by a test described later was used as a measure of the resistance to scratching and cracking of the glass surface. The crack initiation load of the strengthened glass article of the present invention is 3.9 kgf (kilogram force) or more, and can be increased to 4 kgf or more, to 5 kgf or more in some cases, and even to 5.2 kgf or more.

The chemical strengthening of the glass composition is described below.

(Conditions of Chemical Strengthening and Compressive Stress Layer)

Chemical strengthening of the glass composition of the present invention can be performed by bringing the glass composition containing sodium into contact with a molten salt containing monovalent cations, preferably potassium ions, having an ionic radius larger than that of sodium ions, so as to allow ion exchange to take place between sodium ions in the glass composition and the monovalent cations in the form of replacement of the sodium ions by the monovalent cations. Thus, a compressive stress layer having a surface compressive stress is formed.

A typical example of the molten salt is potassium nitrate. A molten salt mixture of potassium nitrate and sodium nitrate also can be used, but it is preferable to use potassium nitrate alone because it is difficult to control the concentration of a molten salt mixture.

The surface compressive stress and the depth of the compressive stress layer of a strengthened glass article can be controlled not only by the glass composition of the article but also by the temperature of the molten salt and the treatment time in the ion exchange treatment.

It is possible to obtain a strengthened glass article having a compressive stress layer with a very high crack initiation load, a moderately large thickness, and a moderately high surface compressive stress by bringing the glass composition of the present invention into contact with a molten salt of potassium nitrate. Specifically, it is possible to obtain a strengthened glass article having a compressive stress layer with a surface compressive stress of 900 MPa or more and a depth of 25 μm or more, and in addition, with a crack initiation load of 3.9 kgf or more, further 5 kgf or more, and even 6 kgf in some cases.

Since this strengthened glass article has a very high crack initiation load, its surface is resistant to cracking and scratching and has a strength suitable for use as a cover glass of a display.

It is also possible to obtain a strengthened glass article having a compressive stress layer with a very high surface compressive stress and a very large thickness by bringing the glass composition of the present invention into contact with a molten salt of potassium nitrate. Specifically, it is possible to obtain a strengthened glass article having a compressive stress layer with a surface compressive stress of 1000 MPa or more, even 1200 MPa or more or 1400 MPa or more in some cases, and a thickness of 30 μm or more, even 40 μm or more or 50 μm or more in some cases.

Since this strengthened glass article has a very high surface compressive stress, its surface is resistant to scratching. In addition, since the strengthened glass article has a compressive stress layer with a very large thickness, even if the surface has a scratch, the scratch is less likely to develop into the glass article due to the presence of the compressive stress layer and thus is less likely to damage the strengthened glass article. This strengthened glass article has a strength suitable for use as a cover glass of a display.

According to the present invention, it is possible to provide a glass composition having a relatively low T₄, suitable for production by the float process, and advantageous in forming glass into a thin glass sheet for use as a cover glass of a display.

The strengthened glass article obtained by chemically strengthening the glass composition of the present invention is suitable for use as a cover glass of a liquid crystal display, an organic EL display, a touch-panel display, or the like for an electronic device. It should be noted that the glass composition of the present invention does not necessarily have to be subjected to chemical strengthening treatment, and the untreated glass composition also can be used as a substrate for an electronic device or the like.

EXAMPLES

Hereinafter, the present invention will be described in more detail by way of examples and comparative examples.

(Preparation of Glass Compositions)

As commonly available glass raw materials such as silica, titanium oxide, alumina, sodium carbonate, potassium carbonate, basic magnesium carbonate, calcium carbonate, strontium carbonate, and barium carbonate were used to prepare glass formulations (batches) having the glass compositions shown in Tables 1 to 6. In some examples, sodium sulfate was used instead of sodium carbonate. In Comparative Examples 8 and 9, zirconium oxide, boron oxide, and tin (IV) oxide were further added to the glass formulations. The batches thus prepared were each put into a platinum crucible and heated in an electric furnace at 1550° C. for 4 hours. Thus, a molten glass was obtained. Next, the molten glass was poured on an iron plate for cooling to obtain a glass plate. Next, this glass plate was again placed in the electric furnace at 600° C. for 2 hours. Then, the furnace was turned off to slowly cool the glass plate to room temperature. Thus, a glass sample was obtained.

For each glass sample, the glass transition temperature Tg, the glass softening point Ts, the working temperature T₄, the melting temperature T₂, the liquidus temperature TL, the thermal expansion coefficient α, the density d, and the Young's modulus E were measured.

The glass transition temperature Tg and the thermal expansion coefficient α were measured using a differential thermal analyzer (Thermoflex TMA 8140, manufactured by Rigaku Corporation). The working temperature T₄ and the melting temperature T₂ were measured by a platinum ball pulling-up method. The density d was measured by an Archimedes method. The Young's modulus E was measured according to JIS (Japanese Industrial Standards) R 1602.

The liquidus temperature TL was measured in the following manner. The glass sample was pulverized and sieved. Glass particles that passed through a 2380-μm mesh sieve but retained on a 1000-μm mesh sieve were obtained. These glass particles were immersed in ethanol and subjected to ultrasonic cleaning, followed by drying in a thermostat. 25 g of the glass particles were placed in a platinum boat having a width of 12 mm, a length of 200 mm and a depth of 10 mm so as to obtain a measurement sample with a constant thickness. This platinum boat was placed in an electric furnace (a temperature gradient furnace) with a temperature gradient from about 850 to 1200° C. for 24 hours. Then, the measurement sample was observed using an optical microscope with a magnification of 100, and the highest temperature in a region where devitrification was observed was determined to be the liquidus temperature of the sample. In all Examples and Comparative Examples, glass particles in the measurement samples were fused together to form rods in the temperature gradient furnace.

(Production of Strengthened Glass)

The glass sample thus obtained was cut into pieces of 25 mm×35 mm. Both surfaces of each piece were polished with alumina abrasive grains and further mirror-polished with cerium oxide abrasive grains. Thus, two or more 5 mm-thick glass blocks having surfaces with a surface roughness Ra of 2 nm or less (a surface roughness Ra according to JIS B 0601-1994) were obtained for each composition. These glass blocks were immersed in a molten salt of potassium nitrate at predetermined temperatures ranging from 380° C. to 420° C., respectively, for 4 to 8 hours so as to allow ion exchange (I/E) to take place and thus to chemically strengthen the glass blocks. After the chemical strengthening treatment, the glass blocks were washed with hot water at 80° C. Thus, strengthened glass blocks were obtained.

In order to reduce the thermal shock applied to the glass blocks, they were preheated before being immersed in the molten salt and were slowly cooled after being immersed in the molten salt (that is, after being removed from the molten salt). Preheating was performed by placing the glass blocks in a space above the level of the molten salt in a container for 10 minutes. Slow cooling was also performed in the same manner as preheating. This slow cooling also has the effect of returning the molten salt remaining on the removed glass blocks as much as possible to the molten salt container.

For the strengthened glass blocks thus obtained, the surface compressive stress CS and the compression depth (the depth of the compressive stress layer) DOL were measured using a surface stress meter “FSM-6000” manufactured by Orihara Industrial Co., Ltd. Tables 1 to 6 collectively show the results.

(Evaluation of Crack Initiation Load Rc)

For some of the strengthened glass blocks obtained as described above, the crack initiation load was evaluated. The crack initiation load was calculated in the following manner using a Vickers hardness tester manufactured by Akashi Corporation. First, a Vickers indenter was pressed against the surface of the glass sample and applied a load of 1 kgf thereto for 15 seconds. 5 minutes after removal of the load, the number of cracks emanating from the corners of a square indentation on the surface of the glass sample was counted. This counting was repeated 10 times, and the total number of cracks was divided by 40, which is the total number of the corners of the indentation for 10 times, so as to calculate the crack occurrence probability P. The level of the load applied was increased to 2 kgf, 5 kgf, 10 kgf, and 20 kgf step by step, and the crack occurrence probability P was calculated at each of the loads in the same manner as described above. Thus, the two adjacent loads WH and WL, between which the probability of 50% (P=50%) occurred, and the crack occurrence probabilities PH and PL at these two adjacent loads (PL<50%<PH) were obtained. The load at which a straight line connecting two points (WH, PH) and (WL, PL) passed through the point of P=50% was obtained and defined as the crack initiation load Rc. Tables 1 to 6 collectively show the results.

In most Examples, the glass transition temperatures Tg were 610° C. or less and the working temperatures T₄ were 1100° C. or less. In some Examples, the melting temperatures T₂ measured were 1550° C. or less. In many Examples, the differences T₄-TL each obtained by subtracting the liquidus temperature TL from the working temperature T₄ were −1° C. or more. In Examples, the densities d were 2.48 to 2.52 g·cm⁻³.

In all Examples, strengthened glass articles each having a compressive stress layer with a very high surface compressive stress (1100 MPa or more) and a moderately large thickness (25 μm or more) and strengthened glass articles each having a compressive stress layer with a very large thickness (30 μm or more) and a moderately high surface compressive stress (900 to 1100 MPa) could be obtained. In some Examples, strengthened glass articles each having a compressive stress layer with a very high surface compressive stress (1000 MPa or more) and a very large thickness (30 μm or more) could be obtained. In some other Examples, strengthened glass articles each having a compressive stress layer with an extremely high surface compressive stress (1200 MPa or more or 1400 MPa or more) and strengthened glass articles each having a compressive stress layer with an extremely large thickness (40 μm or more or 50 μm or more) and a moderately high surface compressive stress (900 to 1100 MPa) could be obtained.

Furthermore, in all Examples, strengthened glass articles each having a compressive stress layer with a surface compressive stress of 900 MPa or more and a depth of 25 μm or more, and in addition, with a high crack initiation load (3.9 kgf or more) could be obtained. In some of them, strengthened glass articles each having a compressive stress layer with a very high crack initiation load of 5 kgf or more, even 6 kgf or more in some cases could be obtained.

By contrast, in Comparative Examples 1 to 5, the surface compressive stresses were less than 900 MPa.

In Comparative Examples 6 to 9, no strengthened glass article having a compressive surface layer satisfying all of the requirements: a crack initiation load of 3.9 kgf or more; a surface compressive stress of 900 MPa or more; and a thickness of 2.5 μm or more, could be obtained.

TABLE 1 Examples 1 2 3 4 5 6 7 8 9 10 11 12 13 Composition SiO₂ 58.0 58.3 59.0 60.0 60.0 60.0 60.0 60.5 60.5 60.5 60.5 60.5 60.5 mol % ZrO₂ 0 0 0 0 0 0 0 0 0 0 0 0 0 TiO₂ 0 0 0 0 0 0 0 0 0 0 0 0 0 B₂O₃ 0 0 0 0 0 0 0 0 0 0 0 0 0 Al₂O₃ 9.7 9.8 11.5 11.0 11.5 11.5 12.0 8.1 8.6 8.8 9.0 9.1 9.2 MgO 10.4 10.1 8.2 7.5 8.2 8.2 7.2 9.5 9.0 9.1 9.1 8.5 7.9 CaO 0.4 0.6 0.3 0 0.3 0.3 0.3 0.4 0.4 0.4 0.4 0.4 0.4 SrO 0.5 0 0 0 0 0 0 0 0 0 0 0 0 Na₂O 20.5 20.8 18.5 21.5 17.5 18.5 18.5 21.0 21.0 21.0 20.8 21.2 21.8 K₂O 0.5 0.4 2.5 0 2.5 1.5 2.0 0.5 0.5 0.2 0.2 0.3 0.2 SnO₂ 0 0 0 0 0 0 0 0 0 0 0 0 0 R₂O 21.0 21.2 21.0 21.5 20.0 20.0 20.5 21.5 21.5 21.2 21.0 21.5 22.0 TL/° C. 1071 1030 1037 967 1038 1022 1005 1020 1009 1008 1047 995 1026 T₂/° C. 1474 1483 1570 1566 1602 1597 1610 1486 1501 1508 1516 1514 1512 T₄/° C. 1074 1079 1140 1129 1159 1153 1165 1071 1082 1086 1092 1093 1092 Tg/° C. 577 579 587 589 596 600 596 554 558 564 573 563 564 α/×10⁻⁷° C.⁻¹ 106 110 109 104 106 103 105 108 107 106 104 106 106 d/q · cm⁻³ 2.51 2.50 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.49 E/GPa 73.8 73.3 74.1 72.3 74.4 74.2 74.1 71.8 71.9 72.1 72.5 72.1 71.7 Highest Rc/kgf 4.9 5.3 4.8 5.3 4.9 5.2 5.3 5.6 5.7 5.8 5.2 5.9 5.5 crack CS/MPa 1122 1070 1155 1106 1174 1224 1187 952 960 1046 1000 988 975 initiation DOL/μm 27.7 30.3 32.8 27.3 31.7 28.3 28.7 28.1 26.4 27.2 31.4 26.2 27.0 load I/E 420° C. 420° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 420° C. 400° C. 400° C. conditions 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr CS CS/MPa 1108 1070 1134 1091 1123 1181 1076 — — 1016 1000 — 1030 ≧1000 MPa DOL/μm 33.9 30.3 42.0 33.4 38.3 36.0 41.3 — — 33.3 31.4 — 29.6 and I/E 420° C. 420° C. 420° C. 400° C. 420° C. 420° C. 420° C. — — 400° C. 420° C. — 380° C. DOL conditions 6 hr 4 hr 4 hr 6 hr 4 hr 4 hr 4 hr 6 hr 4 hr 8 hr ≧30 μm DOL CS/MPa 1259 1323 1155 1106 1174 1224 1187 1031 1050 1046 1109 988 1030 ≧25 μm DOL/μm 28.0 27.0 32.8 27.3 31.7 28.3 28.7 28.5 26.1 27.2 26.2 26.2 29.6 and I/E 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 380° C. 380° C. 400° C. 380° C. 400° C. 380° C. particularly conditions 6 hr 6 hr 4 hr 4 hr 4 hr 4 hr 4 hr 8 hr 8 hr 4 hr 8 hr 4 hr 8 hr high CS CS CS/MPa 1082 1020 1051 920 1033 1087 1036 922 936 943 925 975 955 ≧900 MPa DOL/μm 39.2 48.6 59.4 52.0 56.5 50.9 58.4 39.8 37.4 45.0 44.3 37.0 38.2 and I/E 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 400° C. 400° C. 420° C. 420° C. 400° C. 400° C. particularly conditions 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr deep DOL

TABLE 2 Examples 14 15 16 17 18 19 20 21 22 23 24 25 26 Composition SiO₂ 60.5 61.0 61.0 61.0 61.0 61.0 61.0 61.8 61.8 61.8 62.0 62.4 60.0 mol % ZrO₂ 0 0 0 0 0 0 0 0 0 0 0 0 0 TiO₂ 0 0 0 0 0 0 0 0 0 0 0 0 0 B₂O₃ 0 0 0 0 0 0 0 0 0 0 0 0 0 Al₂O₃ 9.2 8.5 8.5 8.5 8.8 11.5 11.5 8.3 8.3 8.4 10.5 11.3 9.3 MgO 7.9 8.6 9.1 8.9 8.3 7.2 7.2 8.5 8.2 7.9 7.5 7.2 9.3 CaO 0.4 0.4 0.4 0.4 0.4 0.3 0.3 0.4 0.7 0.4 0 0.3 0.4 SrO 0 0 0 0 0 0 0 0 0 0 0 0 0 Na₂O 21.3 20.9 20.8 21.0 21.0 17.5 18.5 20.8 20.8 21.3 20.0 18.8 21.0 K₂O 0.7 0.6 0.2 0.2 0.5 2.5 1.5 0.2 0.2 0.2 0 0 0 SnO₂ 0 0 0 0 0 0 0 0 0 0 0 0 0 R₂O 22.0 21.5 21.0 21.2 21.5 20.0 20.0 21.0 21.0 21.5 20.0 18.8 21.0 TL/° C. 1097 999 1007 922 1033 1011 996 984 965 974 873 974 1013 T₂/° C. 1515 1510 1513 1498 1518 1625 1620 1525 1518 1523 1612 1650 1512 T₄/° C. 1072 1087 1088 1071 1094 1173 1168 1094 1093 1096 1152 1181 1091 Tg/° C. 557 555 563 566 558 591 596 556 557 553 594 612 574 α/×10⁻⁷° C.⁻¹ 109 107 105 106 107 106 103 107 109 106 110 95 105 d/q · cm⁻³ 2.49 2.49 2.49 2.49 2.49 2.48 2.48 2.48 2.49 2.49 2.48 2.47 2.49 E/GPa 71.8 71.8 72.0 71.9 71.8 74.1 73.9 71.7 72.1 71.4 72.5 73.8 72.6 Highest Rc/kgf 6.0 6.0 5.9 5.9 6.0 5.4 5.7 6.4 6.9 6.4 5.6 6.2 5.6 crack CS/MPa 960 984 1044 1024 955 1187 1173 944 993 928 1101 1208 1041 initiation DOL/μm 28.5 28.4 25.5 26.0 27.4 32.9 29.6 25.4 25.0 26.5 26.4 30.3 30.0 load I/E 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 420° C. 420° C. conditions 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr CS CS/MPa 1027 — 1014 990 — 1112 1111 — — — 1009 1208 1041 ≧1000 MPa DOL/μm 30.1 — 31.2 31.8 — 42.0 37.2 — — — 34.0 30.3 30.0 and I/E 380° C. — 400° C. 400° C. — 420° C. 420° C. — — — 420° C. 420° C. 420° C. DOL conditions 8 hr 6 hr 6 hr 4 hr 4 hr 4 hr 4 hr 4 hr ≧30 μm DOL CS/MPa 1027 984 1044 1100 955 1187 1173 944 993 980 1101 1251 1193 ≧25 μm DOL/μm 30.1 28.4 25.5 24.5 27.4 32.9 29.6 25.4 25.0 30.5 26.4 28.7 27.5 and I/E 380° C. 400° C. 400° C. 380° C. 400° C. 400° C. 400° C. 400° C. 400° C. 380° C. 400° C. 400° C. 400° C. particularly conditions 8 hr 4 hr 4 hr 8 hr 4 hr 4 hr 4 hr 4 hr 4 hr 8 hr 4 hr 6 hr 6 hr high CS CS CS/MPa 947 905 908 922 933 1072 1031 933 951 900 980 1186 1011 ≧900 MPa DOL/μm 40.3 35.2 44.6 39.6 38.7 59.4 53.7 35.9 35.3 37.5 48.1 42.9 42.5 and I/E 400° C. 420° C. 420° C. 420° C. 400° C. 420° C. 420° C. 400° C. 400° C. 400° C. 420° C. 420° C. 420° C. particularly conditions 8 hr 4 hr 8 hr 6 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr deep DOL

TABLE 3 Examples 27 28 29 30 31 32 33 34 35 36 37 38 39 Composition SiO₂ 57.0 57.0 57.0 60.0 61.0 58.0 58.0 58.0 58.0 58.0 58.3 58.5 58.5 mol % ZrO₂ 0 0 0 0 0 0 0.5 0 0 0 0 0 0 TiO₂ 0 0 0 0 0 0 0 0.5 0 0 0 0 0 B₂O₃ 0 0 0 0 0 0 0 0 0 0 0 0 0 Al₂O₃ 12.0 11.0 10.0 7.8 8.5 9.7 9.7 9.7 9.7 11.5 9.8 9.7 10.0 MgO 7.2 9.6 12.0 10.8 8.9 10.9 10.4 10.4 10.3 9.6 9.9 10.1 9.8 CaO 0.3 0.4 0.5 0.4 0.4 0.4 0.4 0.4 1.0 0.4 0.8 0.4 0.4 SrO 0 0 0 0 0 0 0 0 0 0 0 0 0 Na₂O 23.5 22.0 19.0 20.2 21.0 20.5 20.5 20.5 20.5 17.5 20.8 21.0 21.0 K₂O 0 0 1.5 0.8 0.2 0.5 0.5 0.5 0.5 3.0 0.4 0.3 0.3 SnO₂ 0 0 0 0 0 0 0 0 0 0 0 0 0 R₂O 23.5 22.0 20.5 21.0 21.2 21.0 21.0 21.0 21.0 20.5 21.2 21.3 21.3 TL/° C. 971 1026 1104 1058 1002 1074 1096 1064 1037 1076 1056 1006 1020 T₂/° C. 1501 1483 1474 1472 1511 1480 1481 1471 1467 1552 1479 1488 1497 T₄/° C. 1099 1083 1074 1061 1087 1073 1074 1074 1072 1130 1078 1079 1086 Tg/° C. 579 587 589 557 560 580 588 577 571 594 578 578 580 α/×10⁻⁷° C.⁻¹ 108 107 111 108 106 105 106 106 101 111 112 105 107 d/q · cm⁻³ 2.50 2.50 2.51 2.49 2.49 2.50 2.51 2.51 2.51 2.50 2.50 2.50 2.50 E/GPa 72.5 73.3 74.6 72.3 71.9 73.4 74.2 73.6 73.7 75.0 75.1 73.0 73.0 Highest Rc/kgf 5.4 4.8 3.9 5.1 5.9 4.5 4.2 4.9 5.9 4.2 5.7 5.0 5.0 crack CS/MPa 1220 1161 1193 886 963 1110 1186 1126 1093 1249 1074 1033 1089 initiation DOL/μm 28.5 30.9 28.7 35.1 35.6 30.1 29.1 29.6 28.1 31.7 30.1 30.9 31.8 load I/E 400° C. 420° C. 420° C. 420° C. 400° C. 420° C. 420° C. 420° C. 420° C. 400° C. 420° C. 420° C. 420° C. conditions 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr CS CS/MPa 1064 1161 1179 1001 — 1110 1180 1114 1075 1062 1074 1033 1089 ≧1000 MPa DOL/μm 39.1 30.9 35.1 32.7 — 30.1 35.6 36.3 34.4 39.4 30.1 30.9 31.8 and I/E 420° C. 420° C. 420° C. 400° C. — 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. DOL conditions 4 hr 4 hr 6 hr 8 hr 4 hr 6 hr 6 hr 6 hr 4 hr 4 hr 4 hr 4 hr ≧30 μm DOL CS/MPa 1220 1318 1356 1011 963 1280 1254 1243 1476 1249 1389 1243 1251 ≧25 μm DOL/μm 28.5 28.3 28.1 28.3 35.6 26.9 25.7 27.7 24.7 31.7 26.3 27.8 28.3 and I/E 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. particularly conditions 4 hr 6 hr 6 hr 6 hr 4 hr 6 hr 6 hr 6 hr 6 hr 4 hr 6 hr 6 hr 6 hr high CS CS CS/MPa 1034 1141 1173 904 922 1070 1164 1104 1053 1020 1034 1000 1049 ≧900 MPa DOL/μm 55.2 43.7 40.5 42.8 39.6 46.0 41.2 41.9 39.7 55.7 42.6 43.7 45.0 and I/E 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. particularly conditions 8 hr 8 hr 8 hr 6 hr 6 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr deep DOL

TABLE 4 Examples 40 41 42 43 44 45 46 47 48 49 50 51 52 Composition SiO₂ 59.0 59.0 59.5 59.8 60.0 60.0 60.0 60.0 60.0 60.5 60.0 60.0 60.0 mol % ZrO₂ 0 0 0 0 0 0 0 0 0 0 0 0 0 TiO₂ 0 0 0 0 0 0 0 0 0 0 0 0 0 B₂O₃ 0 0 0 0 0 0 0 0 0 0 0 0 0 Al₂O₃ 9.5 10.0 10.0 9.5 9.0 9.1 9.3 9.5 9.5 9.0 9.3 8.8 8.3 MgO 9.8 9.6 9.6 9.6 9.6 10.1 10.3 9.2 10.1 9.7 9.8 10.3 10.3 CaO 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.3 0.4 0.8 0.4 0.4 0.4 SrO 0 0 0 0 0 0 0 0 0 0 0 0 0 Na₂O 21.0 20.5 20.0 20.3 20.5 19.4 19.4 20.5 19.5 18.8 20.4 20.2 21.0 K₂O 0.3 0.5 0.5 0.5 0.5 1.0 0.6 0.5 0.5 1.2 0.1 0.3 0 SnO₂ 0 0 0 0 0 0 0 0 0 0 0 0 0 R₂O 21.3 21.0 20.5 20.8 21.0 20.4 20.0 21.0 20.0 20.0 20.5 20.5 21.0 TL/° C. 1013 1031 1032 921 915 1001 1042 975 1051 927 1028 1042 1035 T₂/° C. 1494 1512 1529 1517 1506 1517 1536 1523 1530 1522 1517 1504 1483 T₄/° C. 1081 1095 1104 1095 1086 1094 1099 1098 1102 1100 1094 1083 1068 Tg/° C. 574 581 585 576 571 572 579 576 585 575 579 574 565 α/×10⁻⁷° C.⁻¹ 107 106 105 102 108 106 104 106 104 111 104 105 106 d/q · cm⁻³ 2.50 2.50 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.49 E/GPa 72.7 73.2 73.4 73.0 72.6 73.2 73.2 72.7 73.4 73.3 72.9 72.8 72.2 Highest Rc/kgf 5.2 5.1 5.2 5.3 5.4 5.1 5.1 5.3 5.2 6.0 5.4 5.2 5.4 crack CS/MPa 1014 1038 1062 1015 983 1012 1042 989 1070 1005 1075 1049 1008 initiation DOL/μm 32.7 32.7 30.7 32.6 33.4 33.2 30.3 33.5 29.2 31.3 28.4 28.4 28.4 load I/E 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. conditions 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr CS ≧1000 CS/MPa 1014 1038 1062 1015 1143 1012 1042 1140 1050 1005 1061 1035 1148 MPa and DOL/μm 32.7 32.7 30.7 32.6 33.5 33.2 30.3 30.2 36.1 31.3 34.8 34.8 30.0 DOL ≧30 I/E 420° C. 420° C. 420° C. 420° C. 400° C. 420° C. 420° C. 400° C. 420° C. 420° C. 420° C. 420° C. 400° C. μm conditions 4 hr 4 hr 4 hr 4 hr 8 hr 4 hr 4 hr 6 hr 6 hr 4 hr 6 hr 6 hr 8 hr DOL ≧25 CS/MPa 1213 1236 1246 1203 1161 1180 1233 1140 1243 1210 1186 1200 1169 μm and DOL/μm 28.3 29.1 28.4 28.7 29.1 29.5 26.9 30.2 26.9 26.5 26.3 27.0 25.8 particularly I/E 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. high CS conditions 6 hr 6 hr 6 hr 6 hr 6 hr 6 hr 6 hr 6 hr 6 hr 6 hr 6 hr 6 hr 6 hr CS ≧900 CS/MPa 1079 1002 1010 958 921 962 1001 945 1029 963 1045 1019 973 MPa and DOL/μm 46.2 45.0 44.5 45.8 48.6 46.0 42.6 48.2 42.0 43.9 40.2 40.1 40.2 particularly I/E 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. deep DOL conditions 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr

TABLE 5 Examples 53 54 55 56 57 58 59 60 61 62 Composition SiO₂ 60.0 59.0 60.0 60.0 59.0 60.0 60.0 60.0 60.0 59.0 mol % ZrO₂ 0 0 0 0 0 0 0 0 0 0 TiO₂ 0 0 0 0 0 0 0 0 0 0 B₂O₃ 0 0 0 0 0 0 0 0 0 0 Al₂O₃ 8.3 10.3 8.8 7.8 8.3 7.8 8.8 8.3 8.3 10.3 MgO 10.8 9.3 9.8 10.3 10.3 10.8 10.3 10.8 10.3 9.3 CaO 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 SrO 0 0 0 0 0 0 0 0 0 0 Na₂O 20.1 21.0 21.0 20.5 21.5 20.0 20.1 19.5 20.2 20.8 K₂O 0.4 0 0 1.0 0.5 1.0 0.4 1.0 0.8 0.2 SnO₂ 0 0 0 0 0 0 0 0 0 0 R₂O 20.5 21.0 21.0 21.5 22.0 21.0 20.5 20.5 21.0 21.0 TL/° C. 1055 1017 1024 1048 1041 1061 1044 1064 1047 1020 T₂/° C. 1490 1518 1497 1469 1453 1473 1504 1493 1487 1520 T₄/° C. 1072 1099 1079 1059 1052 1062 1084 1075 1073 1100 Tg/° C. 569 586 569 550 554 556 573 566 568 587 α/×10⁻⁷° C.⁻¹ 106 104 105 110 110 109 105 107 107 104 d/q · cm⁻³ 2.49 2.49 2.49 2.50 2.50 2.50 2.49 2.49 2.49 2.49 E/GPa 72.7 73.2 72.4 72.0 72.0 72.3 72.8 73.0 72.6 72.9 Highest crack Rc/kgf 5.1 5.3 5.5 5.2 5.1 5.0 5.2 5.0 5.2 5.3 initiation load CS/MPa 1028 1121 1024 1087 956 947 1007 1001 973 1112 DOL/μm 28.0 29.7 29.2 25.1 32.5 31.6 30.7 30.4 31.6 30.5 I/E 420° C. 420° C. 420° C. 400° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. conditions 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr CS ≧1000 MPa CS/MPa 1160 1105 1007 1087 1107 1088 1000 1001 1011 1112 and DOL DOL/μm 30.1 34.3 36.3 25.1 33.8 33.7 35.2 30.4 33.7 30.5 ≧30 μm I/E 400° C. 420° C. 420° C. 400° C. 400° C. 400° C. 420° C. 420° C. 400° C. 420° C. conditions 8 hr 6 hr 6 hr 6 hr 8 hr 8 hr 6 hr 4 hr 8 hr 4 hr DOL ≧25 μm CS/MPa 1184 1268 1181 1087 1125 1106 1116 1154 1029 1258 and DOL/μm 26.0 27.5 26.6 25.1 29.3 29.2 26.8 28.5 29.2 28.3 particularly I/E 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. high CS conditions 6 hr 6 hr 6 hr 4 hr 6 hr 6 hr 6 hr 6 hr 6 hr 6 hr CS ≧900 MPa CS/MPa 998 1101 994 910 904 924 980 966 938 1092 and DOL/μm 39.6 42.0 41.3 33.6 45.9 38.7 38.0 42.9 44.7 43.1 particularly I/E 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. deep DOL conditions 8 hr 8 hr 8 hr 4 hr 8 hr 6 hr 8 hr 8 hr 8 hr 8 hr

TABLE 6 Comparative Examples 1 2 3 4 5 6 7 8 9 Composition SiO₂ 75.0 70.0 70.0 65.0 60.0 55.0 55.0 66.0 63.0 mol % ZrO₂ 0 0 0 0 0 0 0 0.01 2.04 TiO₂ 0 0 0 0 0 0 0 0 0 B₂O₃ 0 0 0 0 0 0 0 0.6 1.9 Al₂O₃ 6.5 9.5 9.5 11.5 10.0 11.5 15.0 10.3 8.3 MgO 2.5 2.2 2.5 2.5 9.7 12.5 9.7 5.8 3.2 CaO 0 0.3 0 0 0.3 0 0.3 0.6 2.3 SrO 0 0 0 0 0 0 0 0 0 Na₂O 16.0 15.3 18.0 21.0 13.2 21.0 20.0 14.2 15.6 K₂O 0 2.7 0 0 6.8 0 0 2.4 3.4 SnO₂ 0 0 0 0 0 0 0 0.14 0.13 R₂O 16.0 18.0 18.0 21.0 20.0 21.0 20.0 16.6 19.0 TL/° C. 864 877 861 <833 1138 >1200 1053 <890 <878 T₂/° C. 1832 1794 1786 1701 1582 1471 1577 >1613 1561 T₄/° C. 1263 1264 1251 1213 1147 1072 1155 >1131 1122 Tg/° C. 549 557 577 578 583 607 647 606 569 α/×10⁻⁷° C.⁻¹ 85 98 90 98 113 101 97 92 103 d/q · cm⁻³ 2.41 2.44 2.43 2.46 2.49 2.51 2.50 2.46 2.54 E/GPa 69.9 72.1 71.1 71.2 75.0 74.4 76.5 72.9 75.4 Highest Rc/kgf — — — — — 2.7 3.8 5.6 3.2 crack CS/MPa — — — — — 1266 1519 970 1022 initiation DOL/μm — — — — — 25.2 25.7 23.2 17.5 load I/E — — — — — 420° C. 420° C. 380° C. 380° C. conditions 4 hr 4 hr 4 hr 4 hr CS ≧1000 MPa CS/MPa — — — — — 1240 1505 Not 1011 and DOL/μm — — — — — 30.9 31.5 achieved 30.8 DOL ≧30 μm I/E — — — — — 420° C. 420° C. 420° C. conditions 6 hr 6 hr 4 hr DOL ≧25 μm CS/MPa 585 737 775 845 870 1340 1519 970 1011 and DOL/μm 27.5 36.6 28.9 33.1 45.5 27.2 25.7 23.2 30.8 particularly I/E 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 420° C. 380° C. 420° C. high CS conditions 4 hr 4 hr 4 hr 4 hr 4 hr 8 hr 4 hr 4 hr 4 hr CS ≧900 MPa CS/MPa 430 601 612 700 800 1230 1499 901 1011 and DOL/μm 51.2 67.6 53.2 62.2 75.7 35.6 36.3 47.4 30.8 particularly I/E 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. deep DOL conditions 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 4 hr

INDUSTRIAL APPLICABILITY

The present invention can provide a glass composition suitable for production by a float process and suitable for use, for example, in a cover glass for a display. 

The invention claimed is:
 1. A glass composition comprising, in mol %: 56 to 64% SiO₂; 6 to 12% Al₂O₃; 7 to 12% MgO; 0 to 1% CaO; 17 to 24% Na₂O; and 0 to 1.5% K₂O, wherein a total content of Li₂O, Na₂O, and K₂O is in a range of 18.5 to 24%, and the glass composition is substantially free of Li₂O.
 2. The glass composition according to claim 1, wherein a content of Al₂O₃ is 6 to 10 mol %.
 3. The glass composition according to claim 1, wherein a temperature T₄ at which the glass composition has a viscosity of 10⁴ dPa·s is 1100° C. or less.
 4. The glass composition according to claim 1, wherein a temperature T₂ at which the glass composition has a viscosity of 10² dPa·s is 1550° C. or less.
 5. The glass composition according to claim 1, wherein a difference obtained by subtracting a liquidus temperature TL from a temperature T₄ at which the glass composition has a viscosity of 10⁴ dPa·s is −10° C. or more.
 6. The glass composition according to claim 1, consisting essentially of, in mol %: 57 to 64% SiO₂; 0 to 3% B₂O₃; 7 to 11% Al₂O₃; 7 to 12% MgO; 0 to 1% CaO; 19 to 22% Na₂O; 0 to 1.5% K₂O; 0 to 1% TiO₂; 0 to 1% ZrO₂; 0.02% or less total iron oxide in terms of Fe₂O₃; 0.1 to 0.3% SO₃; and 0 to 0.4% SnO₂, wherein a total content of Na₂O and K₂O is in a range of 19 to 22%.
 7. The glass composition according to claim 6, wherein the glass composition is substantially free of B₂O₃.
 8. The glass composition according to claim 6, wherein the glass composition is substantially free of TiO₂.
 9. A glass composition for chemical strengthening, wherein the glass composition for chemical strengthening is the glass composition according to claim 1 and is used in chemical strengthening treatment.
 10. A strengthened glass article comprising a compressive stress layer formed as a surface of the strengthened glass article by bringing the glass composition according to claim 1 into contact with a molten salt containing monovalent cations having an ionic radius larger than that of sodium ions so as to allow ion exchange to take place between sodium ions contained in the glass composition and the monovalent cations.
 11. The strengthened glass article according to claim 10, wherein the compressive stress layer has a surface compressive stress of 900 MPa or more and a depth of 25 μm or more, and in addition, has a crack initiation load of 3.9 kgf or more, the crack initiation load being defined as an indentation load at which cracks emanating from an indentation formed by a Vickers indenter occur with a probability of 50%.
 12. The strengthened glass article according to claim 11, wherein the compressive stress layer has a surface compressive stress of 1000 MPa or more and a depth of 30 μm or more.
 13. A cover glass for a display, the cover glass comprising the strengthened glass article according to claim
 10. 